This invention relates to photoelectrochemical cells and more particularly to cells which have stable anodes responsive to solar radiation.
Much attention is now turning to photoelectrochemical cells as a means of converting optical energy to chemical fuels and/or electricity. The interest in this field has been stimulated by recent work showing that photoanodes fabricated from n-type semiconducting TiO.sub.2, SnO.sub.2, SrTiO.sub.3, KaTaO.sub.3, KTa.sub.0.77 Nb.sub.0.23 O.sub.3, WO.sub.3, and Fe.sub.2 O.sub.3 are stable to photoanodic dissolution in aqueous electrolytes. Metal oxide-based photoelectrochemical cells involving these materials have been shown to convert light to chemical energy in the form of the photoelectrolytic products H.sub.2 and O.sub.2 from H.sub.2 O. These systems, though they are all stable, either respond only to ultraviolet light or have poor current-voltage properties. n-Type semiconductors which a priori have satisfactory properties in terms of energetic requirements (small band gap, good band positions relative to the redox potential of the substrates) are generally found to undergo photoanodic dissolution. That is, though the oxidation of H.sub.2 O may be energetically feasible, the rate of processes leading to O.sub.2 or H.sub.2 O.sub.2 production from H.sub.2 O does not compete with the photoanodic dissolution of the semiconductor. The photoanodic dissolution of n-type semiconductors is viewed as a key problem in the general use of photoelectrochemical devices for synthesis, fuel generation, and electricity production.
Photoelectrochemical cells using n-type CdS or CdSe anodes (band gaps of 2.4 and 1.7 eV, respectively) have long been known to produce a photocurrent when exposed to radiation. However, anodes made of these materials are subject to photoanodic dissolution to yield Cd.sup.2+ ions and chemical S or Se. Other potentially useful semiconductor photoelectrodes CdTe, GaP, GaAs and InP (band gaps of 1.4, 2.24, 1.35 and 1.25 eV, respectively) also respond to large fractions of the solar spectrum, but the irreversible decomposition encountered in their use as photoelectrodes is a serious detriment to their use in practical solar energy systems.
It is, therefore, a primary object of this invention to provide photoelectrochemical cells which are responsive to large segments of the solar energy spectrum and which are not susceptible to photoanodic decomposition.
It is a feature of this invention that only the electrolyte composition need be modified in order to produce the stable, non-decomposing photoanodes of this invention.